Central Heating - how to find the blockage?

[gas chap]

I would.

drain off again, disconnect the return pipe on the boiler (lowest one)

clean out any debris that's in there.

 

[darkwood]

thanks, I'll try it

 

[brumylad]

if pipework is copper might be able to feel for a blockage by feeling for a length of cold pipe while system is running.

Could possibly confirm sludge blockage using a magnet.

 

[darkwood]

For the record, and it may help someone - I found the blockage. There is a gate valve next to the tank feeding the radiator circuit. It was pretty solidly blocked with sludge.
There was a only a very slight pull to a magnet.

Thanks for your advice guys.